In a world where people upgrade their phone every 2-3 years, a 450Mbps LTE chip doesn't make for much of a selling point. No carrier is going to get anywhere close that during the lifetime of the device.
iPhone 7 is expected to come out later this year. That means some early adopters will still have an iPhone 7 well into 2019, based on your stated 2-3 year upgrade cycle.
No carrier is going to get anywhere close to 450Mbps by 2019?
From http://www.cnet.com/news/verizon-to-hold-worlds-first-crazy-fast-5g-wireless-field-tests-next-year/
Verizon to be first to field-test crazy-fast 5G wireless
September 8, 2015
The nation's largest wireless carrier will begin field trials on so-called fifth-generation, or 5G, technology within the next 12 months, Roger Gurnani, chief information and technology architect for Verizon, said in an interview last week. He expects "some level of commercial deployment" to begin by 2017.
How fast is 5G? Verizon's tests have shown a connection speed that is 30 to 50 times faster than our current 4G network, or higher speeds than what Google Fiber offers through a direct physical connection into the home, Gurnani said.